As far as is known at present, fumigants enter the insect
mainly by way of the respiratory system. The entrance to this
system in larvae, pupae and adults is through the spiracles,
which are situated on the lateral surfaces of the body. The
opening and closing of the spiracles are under muscular control.
To enter insect eggs, gases diffuse through the shell (chorion)
of the egg or through specialized "respiratory
channels". It has been shown that some gases may diffuse
through the integument of insects, but at present the comparative
importance of this route for the entry of fumigants is not known.

It is known that the poisoning of an insect by a fumigant is
influenced by the rate of respiration of that insect; any factor
that increases the rate of respiration tends to make the insect
more susceptible.

The practical significance of the more important factors
influencing the toxic action of fumigants is discussed in the
following paragraphs.

EFFECT OF TEMPERATURE

General Effects

The most important environmental factor influencing the action
of fumigants on insects is temperature. In the range of normal
fumigating temperatures from 10 to 35°C, the concentration of a
fumigant required to kill a given stage of an insect species
decreases with the rise in temperature. From the purely
biological standpoint, this is mainly due to the increased rate
of respiration of the insects in response to the rise in
temperature (Sun, 1946). Also, as pointed out previously,
physical sorption of the fumigant by the material containing the
insects is reduced and proportionately more fumigant is available
to attack the insects. Therefore, within the range mentioned,
conditions for successful fumigation improve as the temperature
rises. These conditions are reflected in the schedules for
recommended treatments included in this manual.

Low Temperature Fumigation

At temperatures below 10°C, the situation is more
complicated. Below this point, increased sorption of the gas by
the body of the insect may counterbalance the effects of decrease
in respiration, and also the resistance of insects may be
weakened by the effects of exposure to low temperatures. With
some fumigants, less gas is required to kill certain species as
the temperature is raised or lowered on either side of some point
at which the insects are most tolerant (Moore, 1936; Peters and
Ganter 1935; Bond and Buckland, 1976). However, with others,
toxicity to the insects declines as the temperature falls; for
example, with methyl bromide there is a moderate decrease in
toxicity down to the boiling point and below this temperature
effectiveness drops off sharply so that the amount of gas
required to kill the insects increases dramatically, as shown in
Figure 6.

For the reasons already given in the previous discussion, at
lower temperatures sorption of the fumigant by the infested
material is increased and more fumigant must be applied to
compensate for this. Also, diffusion of a gas is slowed down in
relation to reduction in temperature.

Prefumination and Postfumioation Temperatures

It is important to bear in mind that the results of a
fumigation may be influenced not only by the temperature
prevailing during the treatment, but also by the temperatures at
which the insects are kept before and after treatment.

If the insects have been kept in a cool environment, their
metabolic rate will be low. If they are immediately fumigated at
a higher temperature, their physiological activity may still be
influenced by their previous history, and the uptake of the
poison may not be as great as if they had been kept at the
temperature of fumigation for a long time previous to treatment
(Pradhan and Govindan, 1953-54). These phenomena can be of
practical significance, particularly for certain species of
insects that may go into a state known as diapause (see Howe
(1962) for description of diapause and list of species involved).
For insects in this state, tolerance to some fumigants, e.g.
methyl bromide and phosphine, may be several times greater than
for non-diapausing insects (Bell, 1977 a,b). For other species
not in diapause, toxicity is usually found to be closely
dependent on the temperature of the fumigation (Bond, 1975; Bond
and Buckland, 1976).

A fumigator must have some knowledge of the previous history
of the infested material as well as the species to be treated if
he or she is to apply the recommended fumigation treatments most
effectively. In all treatments, the material should be warmed to
the treatment temperature for several hours to bring the insects
to corresponding physiological activity before fumigating. If
species disposed to the state of diapause are present (e.g. some
members of the order Lepidoptera and the families Dermestidae and
Ptinidae of the order Coleoptera) the dosage and exposure applied
should be increased to a level that will kill the most tolerant
insects.

Under experimental conditions, variations in postfumigation
temperatures have been observed to influence insect mortalities,
but the effects are more complex than those observed in the study
of prefumigation temperatures. However, the net contribution of
the postfumigation temperature effects would not be of sufficient
importance in practice to influence the results of the procedures
recommended in this manual. Reference to the papers of Sun (1946)
and of Pradhan and Govindan (195354) should be made by those
wishing to pursue this aspect of the subject.

Summary of Temperature Effects

From the foregoing discussion it is clear that temperature has
farreaching effects on all the factors governing the successful
outcome of fumigation. In order to clarify the significance of
these effects they may be summarized as follows:

1. For practical purposes, it is increasingly difficult to
kill insects with fumigants as the temperature is lowered to
10°C. Below this point, in progression, various species or
stages may succumb to low temperature or be weakened by it.

2. Adsorption is the most important physical factor
modifying the penetration of fumigants. The amount of gas
physically adsorbed increases as the temperature is lowered,
and it is necessary to add progressively more fumigant to
sustain concentrations free to act on the insects.
Furthermore, because of this inverse effect, at low
temperatures diffusion of the gas into the material is slower
during the treatment, and there is a corresponding decrease
in the rate of desorption afterwards.

3. Chemical reaction of the fumigant with some of the
fumigated material increases as the temperature is raised. If
the residues formed are of significance, it is advisable to
conduct the treatment at as low a temperature as possible,
with due regard for the handicaps to successful results
summarized in paragraphs (1) and (2).

In the light of these three main effects the influence of
temperature in different types of fumigation may be considered:

1. With commodities that are easily penetrated and are not
highly sorptive, fumigation is practicable at relatively low
temperatures with fumigants such as methyl bromide. It will
be noted that some of the schedules of recommended treatments
at the end of this manual include provision for fumigations
at temperatures down to 4°C.

2. Fumigation at temperatures at which the insects are not
active may be advantageous in some quarantine treatments.
There are two principal reasons for this. Firstly, if seeds
or live plants in dormant condition are being fumigated, the
risk of injury is reduced by avoiding the possible
stimulating effects of higher temperatures on physiological
mechanisms. Secondly, if the infesting insects are active
fliers, their chances of escape from the material awaiting
treatment in a cool environment are greatly reduced.

3. With highly sorptive materials, on the other hand, low
temperature fumigation may not be advisable because increased
adsorption of the gas by the commodity may interfere with
penetration. Also, under some conditions, the material may be
hazardous for handling because the adsorbed fumigant is held
longer at low temperatures.

EFFECT OF HUMIDITY

From the present knowledge of insect toxicology, it is not
possible to make any general statements about the influence of
humidity on the susceptibility of insects to fumigants.
Variations in response at certain humidities have been observed
not only between different species subjected to different
fumigants but also between stages of the same species exposed to
a single fumigant. However, variations due to humidity are not so
important in practice as those due to temperature.

The treatments recommended in this manual are adequate for the
range of moisture content and humidity normally encountered.

EFFECT OF CARBON DIOXIDE

Carbon dioxide, in certain concentrations, may stimulate the
respiratory movements and opening of spiracles in insects. A
number of authors have shown that addition of carbon dioxide to
some of the fumigants may increase or accelerate the toxic effect
of the gas (Cotton and Young, 1929; Jones, 1938; Kashi and Bond,
1975; Bond and Buckland, 1978). With each fumigant acting on
different insects, there seems to be an optimum amount of carbon
dioxide needed to provide the best insecticidal results.
Excessive amounts of carbon dioxide tend to exclude oxygen from
insects and thus interfere with the action of the fumigants.

With certain fumigants, such as ethylene oxide and methyl
formate, the addition of carbon dioxide may work to advantage
both by reducing the fire or explosion hazards and by increasing
the susceptibility of the insects. On the other hand, with
fumigants that are nonflammable, the advantages of adding carbon
dioxide may be offset by the extra cost and work required to
handle the additional weight of containers.

The use of carbon dioxide as a "fumigant" introduced
artificially into grain storages or other structures is described
in Chapter 11.

PROTECTIVE NARCOSIS

Some fumigants can produce paralysing effects on insects that
may alter the toxicity of these or other fumigants. In the use of
hydrogen cyanide (HCN) against insects, it has been shown that,
if certain species are exposed to sublethal concentrations before
the full concentration is applied, the resulting fumigation is
less effective than one in which the insects are subjected to the
full concentration from the very beginning (Lindgren, 1938). A
similar protective effect can also occur with the fumigant
phosphine if insects are exposed to excessive concentrations
during a treatment (Winks, 1974a). Also, insects that have been
narcotized by sublethal concentrations of HCN have been found to
be protected from lethal treatments with other fumigants, e.g.
methyl bromide (Bond, 1961) and phosphine (Bond et al, 1969).
This effect has been referred to as "protective
stupefaction" or "narcosis".

Although phosphine itself can narcotize insects it does not,
however, protect them from the action of methyl bromide as does
HCN; in fact, phosphine and methyl bromide can be used together
as a "mixture" to enhance the effectiveness of each
other (Wohigemuth et al, 1976; Bond, 1978).

From the practical point of view the phenomenon of narcosis is
important because it can reduce the effectiveness of certain
fumigants. However, steps can be taken to avoid problems of this
nature:

1. In fumigations with HCN the maximum concentration
attainable from a recommended dosage should be achieved as
soon as possible at the beginning of the treatment.

2. HCN should not be applied with other fumigants such as
methyl bromide or phosphine, if the maximum toxic effect is
to be achieved.

3. Excessive concentrations of phosphine likely to produce
a protective narcosis should not be used.

FLUCTUATIONS IN SUSCEPTIBILITY OF INSECTS

It has often been observed that there may be fluctuations in
the susceptibility of populations of insects to a given poison.
Some of the reasons are known, while others need further
clarification. Two important factors are undoubtedly seasonal
changes in climate and the effect of nutrition. The
susceptibility of insects may be greatly influenced by the
quality of the food they consume. It also has been observed with
some insects that a certain amount of starvation may make them
more, rather than less, resistant to fumigants (Sun, 1946).

In practical work it is well to know that fluctuations in
resistance may occur. The alert operator must always be on the
lookout for any changed conditions that may necessitate
modification of recommended treatments.

COMPARATIVE TOXICITY OF FUMIGANTS

Apart from the influence of the environment, there is a great
variation in susceptibility of different species of insects to
different fumigants. The successive stages of a given species may
also vary greatly in response. Figure 7 illustrates this point.
The data were obtained during an extensive study of the
usefulness of HCN and methyl bromide for the disinfestation of
empty ships (Monro et al, 1952).

Howe and Hole (1966) have shown that these variations in the
susceptibility of stages of Sitophilus qranarius (L.), observed
under practical conditions, are closely confirmed in laboratory
experiments.

A large number of studies have been made under laboratory
conditions to determine the relative susceptibility of insects to
different fumigants. Table 16 (Chapter 14) shows how fumigants
may vary in their toxicity to common species. Bowley and Bell
(1981) have reported on the toxicity of twelve fumigants to three
species of mites infesting grain.

The treatments recommended here are based on laboratory or
field trials that have been confirmed, in many instances, by the
results of successful application in practice. Note that all
recommended treatments refer to specific insects or their stages
or, in some cases, to clearly defined groups of insects. There
is, therefore, no guarantee of the success of any attempts to
apply a treatment outside the limits given in the recommended
schedules.

Many species of insect have the ability to develop resistance
to certain insecticides. With fumigants this problem of
resistance is a matter of increasing concern; in a global survey
of stored grain pests, resistance to both of the major fumigants,
phosphine and methyl bromide, was found in a number of insect
species (Champ and Dyte, 1976). Collections of 849 strains of
insects from 82 countries showed that 20 percent of the insects
had some resistance to phosphine and 5 percent to methyl bromide.
The highest level of resistance (10-12 times normal) was found in
the lesser grain borer Rhyzopertha dominica (F.). It was
concluded from this survey that resistance to fumigants was, as
yet, limited in extent and often at marginal levels, but that it
was of considerable significance as it posed a real threat to the
future use of fumigants as control agents.

Research in laboratories has shown that a number of
destructive stored product insects can develop appreciable
resistance to fumigants. Selection of the granary weevil
(Sitophilus qranarius) has produced a strain with more than
12fold resistance to methyl bromide (Bond and Upitis, 1976). A
strain of the red flour beetle, Tribolium castaneum (Herbst),
developed a 10-fold resistance to phosphine in six generations
(Winks, 1974b).

There is recent evidence, from field studies in India and
Bangladesh, of the development of resistance to phosphine in the
Khapra beetle (Borah and Chalal, 1979) and other stored grain
pests (Tyler et al, 1983).

Resistance to fumigants is of concern because of the great
value of fumigants for pest control and because of the very
limited number of materials available. Even low levels of
resistance in species of insects that are cosmopolitan and easily
transported to other parts of the world could be of serious
consequence.

In view of the importance of resistance to fumigation, a brief
and simplified account of some features of the problem are given
below.

HOW RESISTANCE DEVELOPS

When a population of insects is exposed to an insecticide some
individuals are killed more easily than others. The insects that
are more difficult to kill may survive after the treatment and
produce offspring that are also hard to kill. These insects are
said to be more tolerant because they can withstand above-average
doses of the poison. If a population is repeatedly treated with
the same insecticide and each new generation has increasingly
higher tolerance, a "resistant" strain is produced.
Resistance is a genetic characteristic that is passed on from one
generation to the next.

In the laboratory, resistance is produced by treating a
population to kill most of the insects, breeding the tolerant
survivors to produce a new generation, re-treating and continuing
the process until a resistant strain is obtained. This process is
known as selection for resistance. A number of strains of insects
with resistance to different fumigants have been produced in this
way (Monro et al, 1972; Bond, 1973; Winks, 1974b; Bond and
Upitis, 1976).

In the field, resistance to fumigants can develop in the same
way. In a grain bin, on a cargo ship or any other place where a
resident population of insects is treated over and over again
with the same fumigant, resistance might develop. Insects that
are not killed may produce new generations with increasingly
greater tolerance. Generally, resistance does not develop as
readily in wild populations as in the laboratory because the
selection process .may be irregular and because they may
interbreed with nontreated susceptible insects. However, the fact
that resistance has been discovered in wild populations indicates
the possibility that further resistance may develop where
fumigants are used regularly.

NATURE OF RESISTANCE

Resistance is an inborn characteristic that allows individual
insects to tolerate above average doses of a poison. Resistant
insects usually are similar in appearance and have the same
habits as susceptible insects. Normally, they can only be
distinguished by their ability to tolerate excessive
concentrations of the fumigant. Tests have been designed for
detecting and measuring resistance to fumigants (FAO, 1975; UK,
1980).

An important feature in resistance is the ability to tolerate
the effects of more than one poison. Insects that have resistance
to one fumigant can, in some cases, also be resistant to other
fumigants. This characteristic, known as
"cross-resistance" is demonstrated by the data in Table
5. It can be seen that granary weevils selected with methyl
bromide were also resistant to several other fumigants, and the
levels of cross-resistance were all significant in terms of
practical control. Such cross-resistance was not found, however,
in insects selected with phosphine (Monro et al, 1972; Kem, 1978)
or ethylene dibromide (Bond,1973).

For routine monitoring to detect the initial appearance of
resistance in wild populations of stored product beetles, it is
convenient to use a discriminating dose, which is expected to
kill all susceptible specimens. The dose chosen is that
corresponding to slightly above the LD(99) 9 obtained from the
regression line for susceptible beetles allowing for, in the case
of phosphine, what appears to be inherent variability of
response. Some discriminating concentrations are given in Table
6. Susceptible reference strains must always be included in
discriminating tests.

When using a discriminating test with fumigants it is always
advisable to make provision for abnormal concentrations. If a
concentration is obtained that is less than the discriminating
concentration, this will be revealed by abnormal survival in the
susceptible reference strain. Abnormally high concentrations may
be revealed by the inclusion in the tests of a reference strain
(or species) with slightly greater tolerance to the fumigant than
the susceptible reference strain on which the discriminating dose
is based, approximately x 1.5 for methyl bromide tests and x 2.5
for phosphine tests. An alternative approach is to use three
dosages, one at the discriminating dose, one at the approximate
LD(90) level and the other at an equivalent level above the
discriminating dose.

In regular monitoring for resistance, it should be detectable
even when only a small proportion of resistant individuals is
present. A minimum of 100 insects in two batches of 50 should be
used per sample.

Limited numbers of insects may not be sufficient to detect low
levels of resistance. Therefore, additional samples should be
obtained, if possible. If, however, there is suspicion of serious
resistance (e.g. from failure of treatments) a test with small
numbers (10 to 20) may provide valuable early indication.

The insects are exposed to the discriminating dose for the
appropriate period in the usual way. If all of them are dead at
the end of the posttreatment holding period, the sample can be
classified as "no resistance detectable", and the
medium in which they were held is put into a hot-air oven to
destroy the culture. On the other hand, the presence of surviving
insects at the end of the test should be regarded as prima facie
evidence of resistance and investigated further.

CONFIRMING RESISTANCE

The appearance of unaffected insects in a discriminating test
could be due to the presence of unusually tolerant individuals
from a normal population. Provided that the conditions of
exposure, the physiological state of the insects and the dosages
are consistent, the probability of a single insect in a batch of
100 being unaffected due to chance is less than 0.1 (e.g. less
than once in 10 tests). It is important to determine whether
incomplete response is due to such causes or to genuine
resistance. This can be checked as follows:

1. The test can be repeated using further samples from the
same field population. The chances of adventitious failure to
respond by a single individual in each of successive tests
decline progressively (less than 0.01, 0.001, 0.0001 and so
on). Survival of two or more indviduals throughout is even
less likely. Therefore, the continued appearance of a
proportion of unaffected individuals can be considered as
proof of resistance.

2. Alternatively, the insects which were unaffected in the
discriminating test may be kept and used for breeding a
further generation. If their reaction is actually due to
resistance, it will be found that a substantially larger
proportion of their progeny will fail to respond to the
discriminating dose.

When these tests indicate that a population of insects is
resistant, then extensive testing should be carried out to
determine the degree of resistance present.

WAYS TO AVOID RESISTANCE

Precautions can be taken to reduce the possibility of insects
developing resistance to fumigants.

Perhaps the most effective measure involves alternate control
practices that do not require chemicals. Good sanitation
procedures, proper storage conditions, insect resistant packaging
and all other measures that prevent infestations from developing
can do much to reduce the need for fumigants. Treatments such as
aeration of the commodity, irradiation, temperature extremes,
insect pathogens, etc. as listed in Chapter 1 can also be
employed.

Where fumigants have to be used on a regular basis, close
guard should be kept against control failures. Complete control
of all insects in a treatment is the best insurance against
resistance.

Periodic checks for resistance should be made in areas that
are fumigated regularly. If signs of resistance begin to appear
(as indicated either by control failures or through the test
procedure) then every effort should be made to eradicate the
population. The measures necessary for eradication will vary in
different situations; they may involve a number of procedures
using both chemical and non-chemical means.

Rotation of fumigants may be effective in some instances,
especially if crossresistance is not a problem. For example,
methyl bromide might be used at intervals in a control programme
that relies mainly on phosphine.

One measure that is not advisable in dealing with resistance
problems involves increased dosing. Such practices as doubling
the dose of fumigant to achieve an economic level of control can
magnify the problem unless complete eradication is assured. Any
insects surviving increased doses may develop even higher levels
of resistance than would occur with the normally recommended
treatment.